A Study of the Surviving Fraction of the CosmicRay Radioactive Decay Isotopes ^{10}Be, ^{26}Al, ^{36}Cl and ^{54}Mn as a Function of Energy Using the Charge Ratios Be/B, Al/Mg, Cl/Ar, and Mn/Fe Measured on HEAO3
Abstract
We have used published HEAO3 data on the charge ratios Be/B, Al/Mg, Cl/Ar, and Mn/Fe to examine the decay of the radioactive cosmicray isotopes^{10}Be, ^{26}Al, ^{36}Cl, and ^{54}Mn at energies from ~600 MeV to 20 GeV nucleon^{1}. From these charge data we can obtain the surviving fractions of these isotopes at higher energy with a comparable accuracy to that obtained using directly measured lowenergy isotopic ratios. As a result of the relativistic time dilation of the lifetimes, these higher energy surviving fractions are a probe of propagation conditions outside the disk and into the halo of the Galaxy. The data are interpreted in terms of a simple leakybox propagation model, then a more realistic diffusion model, and finally a still more realistic Monte Carlo diffusion model for the propagation of cosmic rays. Interpreted in terms of a leakybox model, the data for all four isotopes suggest to varying degrees that the cosmicray particles are propagating into regions of lower density away from the disk. The diffusion model allows one to determine that the scale size of this extended propagation region in the Zdirection is 24 kpc. The data are consistent only with weak Galactic winds ~20 km s^{1} or less. Monte Carlo models require cosmicray lifetimes ~2030 × 10^{6} yr and a halo size 23 kpc. The Monte Carlo calculations suggest that it is the scale size of the diffusion coefficient in the Zdirection relative to the halo size that determines the surviving fractions of these radioactive isotopes at high energies rather than the details of the Z dependence of the matter density.
 Publication:

The Astrophysical Journal
 Pub Date:
 October 1998
 DOI:
 10.1086/306224
 Bibcode:
 1998ApJ...506..335W
 Keywords:

 ACCELERATION OF PARTICLES;
 ISM: COSMIC RAYS;
 Acceleration of Particles;
 ISM: Cosmic Rays